This disclosure relates to resonator coupling modulation spectroscopy. In particular, this disclosure relates to resonator coupling modulation spectroscopy for trace-gas analysis.
The increasing analyte selectivity and sensitivity constraints has led to rapid progress in high-resolution spectroscopy. In particular, infrared trace-gas analyzers demonstrate promise as an enabling technology for environmental and medical diagnostics. Mid-infrared (MIR: 3 to 40 μm) and near-infrared (NIR: 0.7 to 3 μm) laser-based spectrometers are widely applicable for sensitive detection due to strong fundamental and harmonic rotational-vibrational transitions in these wavelength regimes. NIR radiation (e.g. 1.65 μm for 2ν3 overtone-band methane detection) near telecommunication wavelengths is particularly well suited for compact and power efficient spectrometers due to potential for monolithic integration of spectrometer components on a silicon platform.
Numerous spectroscopic schemes have been demonstrated with direct laser-absorption spectroscopy most widely utilized for minimal system complexity. However, in the absence of noise-elimination techniques, the presence of baseline signal fluctuations dominate measurement uncertainty resulting in noise-equivalent absorbance limited by laser technical noise and etalon effects (multiple reflections between two reflecting surfaces). In particular, such problems are exacerbated for on-chip spectrometers due to fabrication tolerances resulting in scattering and optical fringes as well as thermal instabilities resulting in measurement drift. Long-term stability of on-chip sensors have generally remained poor (necessitating frequent spectrometer calibration), and techniques for noise cancellation are lacking. In comparison, free-space techniques utilizing noise reduction (e.g. balanced detection, baseline correction and various modulation schemes) have enabled minimum fractional absorptions below 10−5 Hz−1/2, demonstrating potential of improvement beyond current chip-spectrometer standards.
It is therefore desirable to have a system that resolves the problem of traditional wavelength-modulated etalons (via local modulation of resonant transmission) and low-frequency noise (via kHz coupling modulation rates), while utilizing balanced-detection and resonant enhancement to enable highly-sensitive integrated on-chip tunable laser spectrometers.